The proposed studies are designed to elucidate some of the mechanisms by which the enteric food-borne pathogen, Salmonella typhimurium, performs aerobic/anaerobic growth shifts. The experimental models for these studies are formate dehydrogenase (FDH), thiosulfate reductase (TSR), and trimethylamine oxide reductase (TOR). The two forms of FDH which are linked to hydrogen formation and nitrate reduction, respectively, will be isolated from wild-type Salmonella and from four genotypic classes of FDH mutants and characterized with respect to enzyme subunit structure and effectors. Temperature sensitive FDH mutants will be isolated and studied. Selections for oxygen-insensitive and nitrate-insensitive TSR constitutive mutants are planned as are selections for oxygen-insensitive TOR constitutitve mutants. In each case, the nature of the affected component will be determined. The results of the experiments taken together are expected to provide insight as to the mechanism by which oxygen and other electron acceptors regulate the anaerobic electron transport pathways.